This invention relates to a chemical formulation for determining the wettability of various substrates such as solids, semi-solids, and viscous liquids, and more particularly to a method which includes application of the formulation to a suitable substrate to determine the wettability of the same.
As is known, all matter varies in its ability to wet or to be wetted. This characteristic is inherent with the molecular structure of the substance. The standard unit of measurement is the dyne which by definition is approximately the force that one milligram exerts when under the influence of gravity.
Essentially, certain substances are very difficult to wet such as Teflon. Accordingly, not too many liquids will stick to Teflon, and therefore, it is virtually impossible to print or to paint a Teflon surface. The prior art was cognizant of the general problem of wetting surfaces, and there are many excellent articles as well as texts which describe the mechanics of surface tension and the various parameters which will affect the wettability of different materials.
See for example, an article entitled "How Surface Tension Affects Flexographic Printing" by Franklin A. Dick, the inventor herein, presented at the 1978 FTA Annual Forum, Chicago, May 10, 1978. This article describes the problem inherent in selecting inks to be used in printing and methods to determine that the ink will adequately adhere to the surface to be printed.
To achieve adequate adhesion and smoothness, a coating such as a paint or a printing ink must possess an outer-phase surface tension lower than the critical surface tension of the substrate or surface to which it is being applied. Furthermore, the ultimate critical surface tension of the coating should be lower than that of the substrate if direct molecular bonding is to be achieved. This aspect of surface tension in regard to coating a substrate has been thoroughly investigated in the prior art.
See for example a text entitled CONTACT ANGLE-WETTABILITY AND ADHESION published by the American Chemical Association, R. F. Gould, Editor, 1964, Title 43 of Advances in Chemistry, series 43. Essentially, such texts and other articles describe the problems concerned with applying coatings to surfaces in regard to surface tension and other considerations as well. For example, the wettability of a surface is also a function of the relative polarity of the coating with respect to the substrate. Cognizant of such problems, the prior art has devised various tests and methods to determine wettability. A most popular technique employs the spreading by a swab or a brush of mixed organic solvents of known and various surface tensions.
Mixtures of ethyl cellosolve and dimethyl formamide are commonly used. These solvents are supplied in a kit form, which consists of various bottles, each bottle having a specific surface tension, as for example from 30-56 dynes/centemeter. The user then employs the different liquid to bracket the critical surface tension of the substrate in question. Essentially, if the substrate is wetted by a given liquid, the liquid, when applied to the substrate, will exhibit a uniform pattern. If the liquid cannot wet the substrate, it will retract or otherwise constrict. Accordingly, by using this technique, one can determine the critical surface tension (or the "wetting tension") of the substrate and hence prepare an ink or paint which will adhere to the substrate. Such kits are supplied by various companies, one of which is the Pillar Corporation of Milwaukee. Wis.
The technique as described above is adequate for certain materials such as low density polyethylene. In any event, the system is impractical for measuring any surface which has even a slight degree of solubility in the solvents used. A new solution is formed at the interface which confuses and negates conclusive determination of relative substrate wettability.
Solvent blends also vary in polarity and solvent power. The testing kits as are commercially available, as indicated above, also employ water white, transparent liquids which are difficult to view on various substrates because of lack of visual contrast against the substrate surface. The various solvents are also not stable and cannot be used for monitoring the critical surface tension of various inks, adhesives and paints.
Thus while the apparatus is relatively inexpensive, it has many disadvantages, and therefore, the use of the test kit is extremely limited.
Another technique employed in the prior art serves to establish the contact angle that a droplet of a pure liquid displays when placed on a substrate in question. A device known as a goniometer is employed to establish contact angle and by comparative inference will indicate the relative polarity and critical surface tension of the substrate. This method is fairly precise as compared to the above described technique, but the equipment is not portable, is relatively expensive, and requires technical expertise to obtain reliable data.
The system further suffers in the fact that if migrating additives of substrates are soluble in the testing liquids then the test results can be grossly misleading.
Essentially, as one can ascertain, there is a need for an inexpensive and reliable test for determining the wettability of various substances such as ink, paints and adhesives prior to application of the same to a substrate. It is, therefore, an object of the present invention to provide an inexpensive apparatus which will enable one to establish the relative wettability of various solid and semi-solids.
It is a further object of the present invention to provide a formulation which can be emplaced upon a surface to determine the wettability of the surface which formulation is extremely stable, provides good contrast, and possesses various other characteristics to enable one to more reliably determine the wettability of a surface.
Essentially, as will be described, the apparatus possesses a lower solvent power than prior art systems and also is capable of being colored to provide visual contrast during the test procedure. A major aspect of the formulations to be described is that they possess identical neutral relative polarities and identical ph. The formulations also exhibit the same viscosity and volatility. In this manner many of the disadvantages of the prior art are circumvented and the solutions according to this invention are capable of allowing one to measure surface tension of a much wider variety of substances such as solids, semi-solids and viscous liquids.